As an apparatus for non-invasively measuring internal information of a living body such as the head and the breast, there has been proposed an apparatus which makes use of light absorbing characteristics of a living body to obtain the internal information, that is, an apparatus which uses what-is-called diffuse optical tomography (DOT). With such a measuring apparatus, a region of a living body to be measured is irradiated with light from a predetermined irradiation position, light which is propagated while being scattered in an interior of the region is detected at a predetermined detection position, and internal information on the region, that is, information on a light absorbing body such as a tumor etc. present in the interior of the region can be obtained from measured results of intensity, time waveform, etc. It is noted that Patent Document 1 has described a method for measuring a living body by diffuse optical tomography. Further, Non-Patent Documents 1 and 2 have described a method for reconstructing an image by successive approximation based on diffuse optical tomography.